Unraveling the Nature of Vibrational Dynamics in the Metal Halide Perovskite CsPbI 3

Metal halide perovskites, such as the all-inorganic perovskite CsPbI 3 , are widely considered as highly promising for photovoltaic applications. However, fundamental questions surrounding the vibrational dynamics, especially in regard to the nature of dynamics close to the materials' crystallographic phase transitions are not fully understood. In this contribution, we will report studies of the atomic-scale structure and dynamics of CsPbI 3 , using a combination of inelastic and quasielastic neutron scattering techniques together with molecular dynamics simulations. The results unravel a quasi-harmonic and almost temperature-independent lattice dynamics from 10 K all the way up to the orthorhombic-to-cubic phase transition at around 600 K. In the cubic perovskite phase (T > 600 K) of CsPbI 3 , the dynamics are, conversely, highly anharmonic and overdamped, which are due to anharmonic rattling motions of the Cs + ions with the “perovskite cage” and spatially coherent, yet overdamped dynamics of the Pb-I perovskite sub-lattice. Since the overdamping of vibrational dynamics in metal halide perovskites have been suggested to reduce the interaction with charge carriers and hence lead to an increase of the charge carrier lifetime and diffusion lengths, which are critical factors affecting the performance of metal halide perovskites in actual devices, this new insight can be expected to help in the rational design of new metal halide perovskites towards specific applications.